Recently there has been an increase in the demand for electrolytic bismuth plating processes for electroplating bismuth metal as opposed to electrolytic alloy plating processes where bismuth and at least one other metal such as tin, copper and lead, for example, is being electroplated to form a binary, tertiary or quaternary metal alloy deposit. Typically such alloys have included bismuth as a secondary or tertiary component with another metal or metals dominating the alloy. While bismuth electroplating baths and processes had been known for some time such as, for example, the bismuth plating bath disclosed in U.S. Pat. No. 3,256,160 for plating bismuth directly on steel, such baths, in general, have been difficult to work with because they were unstable, such as due to incompatible bath additives or premature breakdown of bath components, had low plating speeds and low percent current efficiencies, thus being overall inefficient and costly to the industry. Low percent current efficiencies typically mean that undesired side reactions occur along with the main reaction during electroplating. In addition low percent current efficiencies lead to low plating speeds.
Bismuth metal is highly desirable in many industries because of its anticorrosion and antiseizure properties. Bismuth has good wear and good fatigue resistance. Bismuth also has the unique feature of expanding upon solidification, thus having the desired property of conformability. The properties of bismuth make it highly desirable as a metal for bearings, such as in internal combustion engines both gasoline and diesel. Bearings, such as journal bearings, require good surface properties since they must slide against mating surfaces without causing wear to either the surface and without “seizing”, i.e., welding to the mating surface. This property typically requires that the metal or alloy is soft and has a relatively low melting point, or contains a low melting point constituent. The metal or alloy also needs to be capable of carrying the load imposed by the mating surface, which is often cyclic in nature, without break-up or fatigue of the bearing. Sufficient hardness is also an important property, thus a suitable metal or alloy ideally must have a proper balance of all of the foregoing properties.
Since the output of recent internal combustion engines, especially diesel engines, tends to be high, metal overlay layers coating the bearings are subject to peeling off due to fatigue as well as other physical stresses. In addition, corrosive organic acids formed in lubricating oil cause corrosion of the overlay layers. Metals or metal alloys which make up the overlay layer on bearings are typically deposited by electrolytic plating, sintering, sputtering, bonding by rolling and casting processes; however, if such processes deposit a porous layer the reliability of the overlay layer becomes compromised and resistance to fatigue and even rate of corrosion may increase.
Accordingly, there is a need for a bismuth electroplating bath which is stable and electroplates uniform bismuth metal deposits at high plating rates, with high percent current efficiency and may be used in the manufacture of bearings where good fatigue, wear and corrosion resistance are desired.